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Re: PostScript (Was: Re: audio, checksums, and trojan horses)

1991-11-13 15:54:08
PostScript is well enough specified. The only thing is that like many
other content-types it needs a subtype  - in this case used for specifying 
the PostScript level i.e. "Level 1" or "Level 2".

Good point.  This could go in the attribute list.  Maybe we should go ahead
and define "Level=n".

This is already provided for in the PostScript Document Structuring 
Conventions. Specifically, a conformant level 2 document must include
the comment

    %%LanguageLevel: 2

in it. Note also that level 2 actually consists of a group of extension
sets. Some interpreters implements only some of the extension sets and
not others. For this reason it is preferable to list the extension sets
actually used in the document rather than mandating full level 2 support,
which is something very few documents actually need. Indication of the
extension sets used is done with the


comment. Currently the values allowed are DPS, CMYK, Composite, and FileSystem.

In addition, there is more to interpreting a document than simply knowing
what extensions are used. Documents vary in their use of fonts and
other system resources. All of this material can also be described in the
document structuring conventions, and a document manager can then choose the
most appropriate interpreter/output-device combination based on this

This is not idle hypothetical chit-chat. As a user, I have multiple displays 
available to me. A given PostScript document will look best when displayed on 
the proper one (does it use color? what fonts does it use? etc.). A good 
document manager will make these choices based on the structuring conventions.

Should we attempt to reproduce in attributes what can be specified in
the structuring conventions? I guess we could, but I don't see the point
in doing it. PostScript is nice in that it already includes an extensible
mechanism for doing this in any PostScript document. Note that this
information in no way changes the actual document itself; it is simply
supplementary information that "describes" the document, making it easier
to process. The processing itself does not change.

I have written a blurb that describes all this stuff and also describes
PostScript security issues. It is being reviewed now, and I hope to post it to 
the list once my stupider mistakes have been corrected.